The Collaborative International Dictionary of English v.0.48:

obsidian \ob*sid"i*an\, n. [L. Obsidianus lapis, so named,
   according to Pliny, after one Obsidius, who discovered it in
   Ethiopia: cf. F. obsidiane, obsidienne. The later editions of
   Pliny read Obsianus lapis, and Obsius, instead of Obsidianus
   lapis, and Obsidius.] (Min.)
   A kind of glass produced by volcanoes. It is usually of a
   black color, and opaque, except in thin splinters.
   [1913 Webster]

   Note: In a thin section it often exhibits a fluidal
         structure, marked by the arrangement of microlites in
         the lines of the flow of the molten mass.
         [1913 Webster]

WordNet (r) 3.0 (2006):

obsidian
    n 1: acid or granitic glass formed by the rapid cooling of lava
         without crystallization; usually dark, but transparent in
         thin pieces